DESCRIPTION Rationale: The standard upper extremity prosthesis has been a cable-based system in which cables attached to the prosthesis wrap around the back to the contralateral shoulder. By manipulating the contralateral shoulder, the user controls the function of the prosthesis, such as gripping or releasing an object, as well as the force that is applied by the prosthesis. While not natural, the user does receive a form of sensory feedback from the amount of tension developed in the cables. In more recent years, myoelectric prostheses have become available. These newer prostheses rely on voluntary contraction of the residual muscles of the amputated limb to control the function of the prosthesis. While myoelectric prostheses are more cosmetically pleasing and provide a greater range of motion than the traditional cable-based system, they lack sensory feedback. Because the hands are key for manipulating the external environment, sensory feedback is critical in the upper extremities. The ideal artificial sensory feedback mechanism would be one that produces the exact same perception as the non-amputated limb. Although the sensory receptors are missing in the amputated limb, the neural pathways that once carried sensory information remain intact and can be excited with electrical stimulation, thus affording an opportunity for providing sensory feedback to the amputee. Objective: The objective of this study is to prove that electrical stimulation applied to the residal nerves in an amputee in a controlled manner can provide sensory feedback that can be modulated and is reproducible. Further, this study aims to demonstrate that the sensations induced by electrical stimulation are stable over time and that their locations can be artificially manipulated without altering stimulus parameters. Numerous hypotheses will be tested through a series of experiments that span a 10 week time period. Research Plan and Methodology: Five subjects will be implanted with nerve cuff electrodes around residual nerves in their arm: the median, radial, ulnar, and musculocutaenous nerves. Stimulus space will be searched in a gross, rapid manner over the first four weeks of the study. The most promising stimulus parameters and the space surrounding them will be tested in more detail during the next 6 weeks. Subjects will be queried for their perceptions to stimulation and how those perceptions change with time or with changes in stimulus parameters. Sensations that the limb has changed position will be studied by having the subject mirror the position with the contralateral, intact limb. Limb positions will be recorded with a Vicon system. In addition to creating a stimulus-to-percept map, which may vary over time, a percept will be singled out for the purpose of artificial relocation. This will be accomplished by stimulating the nerve and inducing a percept that is in disagreement with what the subject sees. Specifically, pressure will be applied to the fingertip of the subject's prosthesis at the same time that the subject's nerve is stimulated with a set of stimulus parameters known to induce a sensation somewhere else. The visual feedback should allow the subject to adjust the location of the perception to the fingertip. Successful relocatio of a sensation may allow a perfect mapping from where a clinician wants a stimulus to be perceived and where the subject actually perceives it. In addition to the data gathered during the study, which will lead to two manuscripts, a software package will exist that allows fast, efficien, and meaningful stimulus optimization at the conclusion of the study. This software will be useful in future studies that incorporate additional stimulus channels or locations. The data obtained in this study will guide a future phase in which a prosthesis is designed to control sensory feedback and the subject's ability to perform tasks of daily living with the sensory feedback is evaluated.